Polycyclic aromatic hydrocarbons (PAHs) are a sort of chemicals structured with binary or multiple aromatic rings, which includes naphthalene, methylnaphthalene, dimethyl naphthalene and etc. PAHs are prevalent in diesel. However, their cetane values are so low that PAHs can hardly combust completely. Moreover, PAHs in the diesel mainly lead to smoke emission of off-gas. According to GB19147-2013, the PAHs content in the automotive diesel oil is limited to 11%. Whereas, PAHs content in the diesel, produced from industrial units operation such as catalytic cracking and coking, is far more than 11%.
For high-PAH-content diesel, especially catalytic cracking diesel, there is a hydroconversion process claimed in CN1156752A. In CN1156752A disclosure, the catalysts containing zeolite are employed in hydroconversion. Two-stage hydrogenation process is utilized for dearomarization, desulfidation and upgrading cetane value. In specific conditions, the catalysts have ring-opening catalytic activity and no activity on cracking reactions. The cetane value of processed diesel will increase by more than 10 units, upgrading the quality of diesel remarkably.
Solvent extraction is also feasible to separate PAHs from diesel. A typical process is disclosed in CN103214332B. In CN103214332B disclosure, amine, ether and ionic liquid are employed as solvent for extraction. Majority of strongly polar PAHs in the diesel can be removed after multi-stage extraction, leading to a huge rise of cetane value. With this method processed, after four-stage extraction, PAHs content in the diesel can decrease to 7% and the removal rate of PAHs can reach to 70%. The processed diesel, generated from catalytic cracking, can be used as raw material of national III diesel.
Besides, adsorption is another feasible way to separate PAHs from diesel.
There is an adsorbent and its preparation method claimed in CN102908991B, which can separate naphthalene from organic gas via adsorption. This adsorbent is supported with activated carbon. A small amount of hexadecyl trimethyl ammonium bromide or aluminum chloride is loaded as assistant. With this adsorbent, the naphthalene in the organic gas can decrease to 45 mg/g under the conditions of maximum pressure 3 MPa and maximum temperature 50° C.
Currently, hydrogenation and solvent extraction are prevalent in the separation PAHs from diesel. However, hydrogenation has disadvantages of high energy consumption and high equipment investment while solvent extraction has disadvantages of long process flow and high energy consumption. Due to disadvantages mentioned above, the current cost of upgrading diesel is high. Hence, the selective adsorption separation of PAHs will be an important complement for diesel-upgrading technologies.